Trans-Impedance Amplifiers (TIAs) are used for current to voltage conversion in communication systems. For example, in optical communication systems, TIAs are used for current to voltage conversion in a power control loop to control a LASER or a Light Emitting Device (LED). The terms “LED” and “LASER” are used interchangeably hereinafter. The power contained in the light emitted by the LED is controlled by adjusting the current supplied to the LED. The TIAs are used in many other applications besides power control in a communication system.
FIG. 1 illustrates a Power Control Loop 100 used in a communication system. Light emitted by a LED 104 is detected by a monitor photo detector 108. The photo detector 108 converts the light into a current 112, the amount of the current 112 being proportional to the power contained in the light. The current 112 is received by a Trans-Impedence Amplifier (TIA) 116, which generates a voltage 120 responsive to the current 112. A comparator circuit 124 compares the voltage 120 to a reference voltage 128. If the voltage 120 is higher than the reference voltage 128, the comparator circuit 124 instructs a laser driver 132 to increase the current supplied to the LED 104. If the voltage 120 is lower than the reference voltage 128, the comparator circuit 124 instructs the Laser Driver 132 to decrease the current supplied to the LED 104. Accordingly, the laser driver 132 adjusts the current supplied to the LED 104.
As will be understood by those skilled in the art, in order to precisely control the LED power, the TIA 116 must maintain its linearity error below a desired percentage. Also, the TIA 116 must have the necessary bandwidth to be responsive to the high-speed output of the LED 104. If the output of the LED 104 is 2.5 Gb/s, but the TIA 116 has a low bandwidth (e.g., 100 MHz bandwidth), the TIA 116's output will contain error. Also, the output current from different photo detectors often vary even when the photo detectors are paired with the same LED because of variations in process and manufacture. Thus, the TIA 116 must be able to operate with the same speed and accuracy for a wide range of input currents from the photo detector 108. Also, as will be understood by those skilled in the art, the parasitic capacitance of the photo detector 108 can vary significantly and can often be as high as 15 pF. The variation of the parasitic capacitance of the photo detector 108 is caused by the reverse bias variation of the photo detector 108 and also caused by the fact that system manufacturers frequently change photo detectors. Thus, the TIA 116 must be tolerant of the variations of the parasitic capacitance.
Several variations of TIAs are currently being used. FIG. 2 illustrates a TIA 200 with a high gain differential amplifier and a negative feedback. A photo detector 204 detects light emitted by a LED (not shown in FIG. 2) and generates a current Iin 208. An operational amplifier 212 is coupled to the photo detector 204. The operational amplifier 212 has a non-inverting input terminal 216, an inverting input terminal 220, and an output terminal 224. A feedback resistor 228 is connected between the output terminal 224 and the inverting input terminal 220 of the operational amplifier 212. A bias voltage Vbias 232 is connected to the non-inverting input terminal 216. The output voltage signal is −Iin*R.
In order for the TIA 200 to have high speed capability (i.e., large bandwidth), the operational amplifier 212 must have a large bandwidth. For example, if a several hundred MHz TIA bandwidth is desired, the operational amplifier 212 must have a bandwidth of several GHz, which is difficult to achieve. Also, when different photo detectors 204 are used, different currents may be generated by the photo detectors even when paired with the same LED. For example, if the output current of the photo detector 204 increases by a factor of 3, the feedback resistor 228 value must be decreased accordingly so that the output voltage of the operational amplifier 212 is not distorted. However, if the feedback resistor 228 becomes too small, the amplifier bandwidth may not be large enough to ensure stability. Thus, adjusting the feedback resistor value to achieve linearity across a desired input current range affects the stability of the circuit. Furthermore, if the photo detector 204 is replaced with another that has a different parasitic capacitance, the operational amplifier 212 may not have enough bandwidth to ensure stability.
FIG. 3 illustrates a TIA 300 with a current mirror. A photo detector 304 generates a current Iin 308 responsive to light emitted by a LED (not shown in FIG. 3). A current mirror circuit is coupled to the photo detector. The current mirror circuit is formed by two NMOS transistors 312 and 316 and a resistor 324. The transistor 312 is supplied with a bias current Ibias 320 from a supply voltage Vdd 322. The transistor 316 is connected to the voltage Vdd 322 via the resistor 324. The gate and the drain of the transistor 312 are shorted. The current flowing into the transistor 312 is equal to (Iin+Ibias). If the transistors 312 and 316 have the same size, the output voltage Vout 328 is equal to Vdd−R*(Ibias+Iin). The signal portion of the output voltage is equal to −R*Iin. However, if the transistor 316's size is X times the transistor 312's size, then Vout 328=Vdd−X*R*(Ibias+Iin) and the signal portion of the output voltage is −X*R*Iin.
The TIA 300 lacks a large bandwidth because the parasitic capacitance (not shown in FIG. 3) of the photo detector 304 is added to the parasitic capacitances of the transistors 312 and 316. Also, while the TIA 300 provides good linearity because the load resistor 322 can be programmed independently, especially if a cascaded structure is used, the bandwidth of the TIA 300 is dependent on the input impedance. To mitigate the variation of the input impedance, and therefore the bandwidth, the current Ibias 320 is added to the input current. However, the addition of the current Ibias 320 causes the current through the load resistor 324 to increase, thereby reducing the available voltage swing for the signal at the output node 328.
Accordingly, there is a need for a TIA that does not suffer from the foregoing disadvantages. There is a need for a high-speed TIA that operates with accuracy in response to a wide range of input currents.